The oestrous cycle is a fundamental biological process in non-human mammals, representing the period when a female is sexually receptive and fertile. This cycle is a recurring series of physiological changes driven by reproductive hormones, which prepare the female’s body for potential mating and pregnancy.
The Oestrous Cycle Explained
The oestrous cycle is a rhythmic pattern of sexual behavior and physiological changes that begins after a female mammal reaches sexual maturity. This cycle is orchestrated by the intricate interplay of hormones, primarily estrogen and progesterone, which fluctuate to prepare the reproductive tract for ovulation and, subsequently, for potential embryo implantation.
Stages of the Oestrous Cycle
The oestrous cycle is divided into distinct phases, each characterized by specific hormonal shifts and physiological events. These four main stages are proestrus, oestrus, metestrus, and diestrus, with some species also experiencing anestrus.
Proestrus marks the preparatory phase, where ovarian follicles begin to develop and produce increasing levels of estrogen. During this time, the female’s reproductive tract, including the uterus and vagina, undergoes changes in preparation for potential mating.
Oestrus, often referred to as “heat” or “standing heat,” is the period of peak sexual receptivity and fertility. Estrogen levels reach their highest point, which triggers ovulation in most species. Females in oestrus are receptive to mating and may actively seek out males.
Following oestrus, metestrus is a transitional phase where the ruptured follicle transforms into a corpus luteum, which begins to produce progesterone. This hormone prepares the uterus for a possible pregnancy by making the uterine lining more secretory.
Diestrus is the luteal phase, characterized by high progesterone levels maintained by the fully functional corpus luteum. The uterus is maintained in a state ready for implantation if conception occurred. If pregnancy does not happen, the corpus luteum eventually regresses, and the cycle prepares to restart. Anestrus is a period of sexual inactivity or rest, observed in some species.
Recognizing Oestrous
Observing behavioral and physical signs is a common way to identify when a female mammal is in oestrus. Behavioral changes often include increased restlessness, vocalization such as bellowing in cows, and an increased interest in pen mates. Females may also exhibit mounting behavior towards other animals, or show receptivity to males by standing to be mounted, a clear sign in many species.
Physical indicators can also be present, such as swelling and reddening of the vulva, which is particularly noticeable in animals like sows and does. Vaginal discharge, which can be clear and mucus-like, is another common sign, often seen hanging from the vulva in cows. Other signs may include frequent urination and, in mares, “winking” of the vulva, which is the eversion of the clitoris.
Variations in Oestrous Cycles
Oestrous cycles display considerable variation across different animal species in terms of frequency and duration. Animals are classified based on their cycling patterns throughout the year.
Monoestrous animals, such as dogs, foxes, wolves, and bears, typically experience only one oestrous cycle per year, often coinciding with a specific breeding season.
Polyestrous animals, like cows, pigs, mice, and rats, exhibit multiple oestrous cycles continuously throughout the year, allowing for more frequent breeding opportunities.
Seasonally polyestrous animals, including horses, sheep, goats, and cats, have multiple cycles but only during certain times of the year, usually influenced by factors like daylight hours. For instance, horses are long-day breeders, cycling in spring and summer, while sheep and goats are short-day breeders, cycling in autumn and winter.
The oestrous cycle in non-human mammals differs from the human menstrual cycle in a significant way: animals with oestrous cycles reabsorb the uterine lining if pregnancy does not occur, whereas humans shed it through menstruation. Furthermore, females in oestrous are generally only sexually receptive during the oestrus phase, in contrast to the continuous sexual receptivity observed in humans.